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CN-121733017-B - Optical lens curved surface polishing method based on femtosecond laser

CN121733017BCN 121733017 BCN121733017 BCN 121733017BCN-121733017-B

Abstract

The invention relates to an optical lens curved surface polishing method based on femtosecond laser, which is applied to the technical field of optical lens precision machining and comprises the core steps of establishing hard synchronous trigger connection of the femtosecond laser and a coaxial white-light OCT, delaying the trigger OCT after laser pulse to collect three-dimensional data, simultaneously carrying out coaxial micro-air flow purging to resist interference, adopting a multi-parameter end point model of a weighted voting mechanism to carry out qualification judgment on machining depth, surface roughness and subsurface damage, dynamically adjusting a 3D galvanometer scanning parameter and a femtosecond laser parameter according to deviation and material removal rate if the machining depth, the surface roughness and subsurface damage are not qualified, and executing predictive thermal management based on OCT reflectivity monitoring. The invention solves four problems of monitoring interference, single quality criterion, efficiency precision unbalance and thermal management hysteresis in the prior art through anti-interference monitoring, multi-parameter intelligent judgment, dynamic self-adaptive execution and thermal management closed loop, and realizes high-precision, high-efficiency and high-qualification rate polishing of the curved surface of the high-end optical lens.

Inventors

  • ZHANG LIQING
  • ZHANG RUI
  • LI KUI
  • ZHANG XIAOSHI

Assignees

  • 云南大学

Dates

Publication Date
20260508
Application Date
20260227

Claims (10)

  1. 1. The optical lens curved surface polishing method based on the femtosecond laser is characterized by comprising the following steps of: S1, fixing an optical lens substrate to be polished on a processing platform, starting a coaxial white-light OCT detection unit, a femtosecond laser and a 3D galvanometer, and establishing hard synchronous trigger connection between a femtosecond laser pulse signal and the coaxial white-light OCT detection unit through a programmable delay circuit; S2, through the hard synchronous trigger connection, delaying for 5-10 mu S after femtosecond laser pulse output, triggering the coaxial white-light OCT detection unit to collect three-dimensional data of a lens to-be-processed area, and simultaneously controlling a coaxial micro-air flow nozzle forming an included angle of 30 DEG+/-5 DEG with a laser path to spray inert gas at a flow rate of 0.3-0.8L/min to blow away scraps of the to-be-processed area, wherein the three-dimensional data comprises depth data, intensity data and spectrum data; S3, inputting the three-dimensional data into a preset multi-parameter end point model, and judging whether a region to be processed meets a preset processing target, wherein the preset processing target comprises a processing depth error of less than or equal to +/-0.3 mu m, a surface roughness Ra of less than or equal to 1nm based on inversion of intensity data, no material phase change characteristic peak or characteristic peak intensity of less than or equal to 5% of a base line in spectrum data, judging that the processing is qualified if the three items are met, executing step S5, judging that the processing is complemented if at least one item is not met, outputting deviation data, and executing step S4; S4, dynamically adjusting scanning parameters of a 3D vibrating mirror and processing parameters of a femtosecond laser according to the deviation data and the material removal rate calculated in real time, wherein if the material removal rate is less than 0.5 mu m/S, the scanning line spacing of the 3D vibrating mirror is reduced to 20-30 mu m, the scanning circle number is increased, if the surface roughness Ra (optical coherence) based on inversion of intensity data is more than 1nm, the repetition frequency of the femtosecond laser is increased to 2MHz, the pulse energy of the femtosecond laser is reduced to 2 mu J, and simultaneously, a thermal management step is executed, namely, the reflectivity change rate of the edge of a processing area is monitored in real time through a coaxial white light OCT (optical coherence tomography) detection unit, if the reflectivity change rate is more than or equal to 3%/ms, the scanning sequence of the 3D vibrating mirror is adjusted, the cold area with the distance of more than or equal to 5mm is preferentially processed, and if the coaxial white light OCT detection unit detects that the local thermal deformation bulge is more than or equal to 0.2 mu m, the energy of the femtosecond laser is reduced to below a preset damage safety threshold and the bulge area is complemented; S5, controlling the 3D galvanometer to move to a next area to be processed, and repeating the steps S2-S4 until polishing of the curved surface of the whole lens is completed; in step S4, the preset thermal damage safety threshold is 80% of the laser damage threshold of the material of the lens substrate to be processed.
  2. 2. The method for polishing a curved surface of an optical lens based on femtosecond laser according to claim 1, wherein in step S1, the delay precision of the programmable delay circuit trigger signal of the hard synchronization trigger connection is ±0.1 μs.
  3. 3. The method for polishing the curved surface of the optical lens based on the femtosecond laser according to claim 1, wherein in the step S2, the inert gas is nitrogen or argon, and the purity is more than or equal to 99.99%.
  4. 4. The method for polishing the curved surface of the optical lens based on the femtosecond laser according to claim 1, wherein in the step S2, the axial resolution of the coaxial white-light OCT detection unit is less than or equal to 5 μm, the transverse resolution is less than or equal to 10 μm, the sampling frequency is more than or equal to 1kHz, the three-dimensional data is preprocessed by a denoising algorithm, and the signal to noise ratio after preprocessing is more than or equal to 35:1.
  5. 5. The method for polishing the curved surface of the optical lens based on the femtosecond laser according to claim 1 is characterized in that an output signal of the multi-parameter end point model is directly transmitted to a control unit of the 3D galvanometer, the control unit adjusts scanning path parameters of the 3D galvanometer in real time according to qualified or complementary processing signals to realize closed loop control of algorithm decision and hardware execution, the multi-parameter end point model adopts a weighted voting mechanism to judge that 1 score is obtained when a single index meets a preset processing target and 0 score is obtained when the single index does not meet the preset processing target, and the weighted score = processing depth error score x 0.4+ surface roughness score x 0.3+ spectral characteristic peak intensity score x 0.3, and only when the weighted score is more than or equal to 0.9, the processing is judged to be qualified.
  6. 6. The method for polishing curved surface of optical lens based on femtosecond laser as recited in claim 1, wherein in step S4, the material removal rate is calculated by the formula And calculating, wherein Deltad is the difference value of depth data acquired by two adjacent times, deltat is the time interval of the two times, the unit is mu m/s, the initial value of the scanning line interval of the 3D galvanometer is 50 mu m, the initial value of the repetition frequency of the femtosecond laser is 1MHz, and the initial value of the pulse energy is 5 mu J.
  7. 7. The method for polishing the curved surface of the optical lens based on the femtosecond laser according to claim 1, wherein in the step S4, when the 3D galvanometer scanning sequence is adjusted, the cooling time of the cold area is more than or equal to 20ms, and the original processing area is returned to be polished continuously after the cooling is finished.
  8. 8. The method for polishing the curved surface of the optical lens based on the femtosecond laser according to claim 1, wherein the method further comprises a preprocessing step, wherein a preset processing mode is selected according to processing requirements, the processing mode comprises a quality priority mode, an efficiency priority mode and a standard mode, if the quality priority mode is selected, the threshold value of the surface roughness Ra in the step S3 is adjusted to be less than or equal to 0.8nm, the monitoring threshold value of the reflectivity change rate in the step S4 is adjusted to be more than or equal to 2%/ms, and if the efficiency priority mode is selected, the threshold value of the processing depth error in the step S3 is adjusted to be less than or equal to + -0.5 mu m, and the monitoring threshold value of the reflectivity change rate in the step S4 is adjusted to be more than or equal to 5%/ms.
  9. 9. The method for polishing curved surface of optical lens based on femtosecond laser as set forth in claim 8, wherein when the efficiency priority mode is selected, the pulse energy of the femtosecond laser in step S4 is adjusted to 4-8 μj, the repetition frequency is adjusted to 0.5-1mhz, and the scanning speed of the 3d galvanometer is adjusted to 600-1000mm/S.
  10. 10. The method for polishing the curved surface of the optical lens based on the femtosecond laser according to claim 1, wherein the material of the optical lens substrate comprises quartz, sapphire or infrared glass, the type of the curved surface of the lens comprises a spherical surface, an aspherical surface or a free curved surface, and the diameter of the lens is in the range of 50-500mm.

Description

Optical lens curved surface polishing method based on femtosecond laser Technical Field The invention relates to the technical field of optical lens precision machining, in particular to a polishing method for an optical lens curved surface based on femtosecond laser, Background In large numerical aperture lithography systems and high-precision optical imaging systems, the application of special surface-shaped (such as aspheric surfaces and free-form surfaces) optical lenses is indispensable for eliminating aberration to ensure imaging quality, and the processing quality of such lenses directly depends on a real-time controllable high-precision polishing technology. Currently, the mainstream lens polishing technology is mainly divided into two types, and has obvious defects: 1. The mechanical polishing technology has the limitations that surface materials are removed through cutting and plastic deformation, multiple grinding tools and abrasive materials are needed to be matched, manual repeated polishing and surface shape detection are needed to be relied on free surface type curved lenses, for example, when sapphire aspheric lenses with the diameters of 200mm are processed, at least 5 polishing and 3 detection are needed, the single processing period exceeds 48 hours, the production efficiency is low (daily average productivity is less than or equal to 2 sheets), the yield is less than 75%, and the requirements of sufficient production cannot be met, meanwhile, the grinding tool loss can cause poor processing consistency, the surface type precision is difficult to break through by 1.2λ (λ=632.8 nm), and the requirements of a photoetching machine on the surface type precision of the lenses cannot be adapted. 2. The non-mechanical polishing technology has the defects that the non-mechanical polishing (such as electrochemical polishing, plasma polishing, liquid jet polishing and the like) is improved on the automation level, but has two major core problems that the cost is high (the single manufacturing cost of plasma polishing equipment is more than 500 ten thousand yuan, the consumable replacement period is short, and the single replacement cost is more than 10 ten thousand yuan), and harmful chemicals such as hydrofluoric acid, nitric acid and the like are needed to be used, about 20L of fluorine-containing wastewater is generated when 100 lenses are processed, the environment is irreversibly polluted, and the current industry trend of green manufacturing is not met. 3. The prior art has the defects that the femtosecond laser is used as a cold working means, and has the advantages of wide material adaptability and small heat affected zone (less than or equal to 1.5 mu m), but the prior art still faces four technical bottlenecks recognized in the industry when realizing batch and high-consistency processing of any surface type optical lenses: The monitoring interference problem is that plasma (duration is about 3-8 mu s) and material scraps generated in the processing process seriously interfere with detection signals of the coaxial OCT, so that the detection signal-to-noise ratio is only 10:1, the depth detection error is more than or equal to 0.5 mu m, and the follow-up closed-loop control loses a reliable data basis; The quality judgment is single, and the prior art only takes the processing depth as the end point judgment basis, can not evaluate the surface roughness (Ra is often more than or equal to 1.8 nm) and subsurface damage (such as 915nm characteristic phase transition peak of quartz lens) on line, thus causing the implicit rejection of the geometric dimension reaching the standard and the optical performance failing, and having low yield; the difficult problems of unbalance of efficiency and precision are that the material removal rate of different areas on a complex curved surface is obvious, the fixed processing parameters (such as 50 μm of scanning line spacing and 5 mu J of laser energy) cannot simultaneously meet the high-precision requirement of a steep section and the high-efficiency requirement of a plane section, and the overall processing efficiency is low (less than or equal to 15mm 2/min); the difficult problem of thermal management hysteresis is that although femtosecond laser is cold processing, under the requirements of high repetition frequency and high efficiency processing, the heat accumulation effect still can cause local micro deformation (the bulge is more than or equal to 0.3 mu m) of a lens, and the traditional thermal management is mostly post-compensation, has a response hysteresis and seriously affects the consistency of the full-aperture surface type. In order to solve the problems, the invention provides a femtosecond laser-3D galvanometer-coaxial OCT collaborative intelligent polishing method, which realizes high-precision, high-efficiency and high-consistency processing of a high-end optical lens through hard synchronous triggering, multi-parameter judgment, dynam